I thought Google used fancy machines to scan all those books, but based on this image I came across while doing some research, maybe not:
If somebody is actually turning all those pages by hand, I hope he has an ample supply of those finger condoms.
Update: According to Google, although many books can be scanned by machine, that involves either some risk of damaging the book or the need to damage it deliberately by cutting off the binding. Google says that many of the books it's scanned were on loan from various libraries, which wanted their books back (intact and with binding included). As a result, it is indeed frequently the case that somebody is actually turning all those pages by hand.
The hand itself is not supposed to be scanned, of course, and Google says the Books team has been alerted to the presence of this particular hand. So those of you who, like me, were interested in this page of Plato's Laws and, also like me, were too cheap to just go buy your own copy of it, may be in luck.